Effect of jet momentum flux and heat density on NOx emission in a flameless gas turbine combustor

Abstract

Experimental testing and validated CFD models have been used to evaluate the performance of the FC4GT flameless gas turbine (GT) combustor operating at a global equivalence ratio of 0.44. The methane-fuelled combustor was tested at atmospheric pressure, modelled at elevated pressures up to 25 bar, and optimised for a combustor pressure of 15 bar. The combustor was analysed for firing rates between 15 kW and 16 MW, which correspond to heat density range between 0.15 MW/m3.bar and 150 MW/m3.bar, respectively. The performance of the combustor was also evaluated with pure hydrogen. Overall, the flow fields while using both fuels, are found to be similar; exhibiting highly uniform spatial (temperature and oxygen) distributions, and low exhaust NOx emissions. Volumetric histograms of temperature and oxygen in the combustor are identified as a powerful analysis tool for quantifying the level of uniformity of the scalar mixing and combustion field, NOx emission, and for determining the combustion regime.The momentum flux of the primary air jet has been identified as a key parameter in controlling NOx emission. Across the power generation mode for GT operation with a heat density of 5–150 MW/m3.bar, the predicted NOx emission (15% O2 dry) for methane was 34–6 ppmv, respectively, and for hydrogen 353–33 ppmv, respectively. At 50 MW/m3.bar, the predicted NOx emission for methane was 17 ppmv, and 146 ppmv for pure hydrogen. This is a significant outcome for a non-premixed combustor considering that the NOx emission of the GE6FA GT engine using partially premixed combustion at 50 MW/m3.bar, is 15 ppmv for methane, and 200 ppmv for 85% hydrogen. Overall, the low NOx emission, for methane and hydrogen, combined with the highly uniform spatial temperature and oxygen distributions, and supported by experimental visualisation provide irrefutable proof that a flameless combustion regime has been achieved. It also demonstrates the plausibility of a non-premixed flameless combustor achieving comparable NOx emission to premixed flames.